The emerging RFID technology employs a Radio Frequency (RF) wireless link and ultra-small embedded computer chips to allow physical objects to be identified and tracked via these wireless “tags”. It functions like a bar code that communicates to the reader automatically without needing manual line-of-sight scanning or singulation of the objects.
The use of RFID tags are quickly gaining popularity for use in the monitoring and tracking of an item. RFID technology allows a user to remotely store and retrieve data in connection with an item utilizing a small, unobtrusive tag. As an RFID tag operates in the radio frequency (RF) portion of the electromagnetic spectrum, an electromagnetic or electrostatic coupling can occur between an RFID tag affixed to an item and an RFID tag reader. This coupling is advantageous, as it precludes the need for a direct contact or line of sight connection between the tag and the reader.
Utilizing an RFID tag, an item may be tagged at a period when the initial properties of the item are known. For example, this first tagging of the item may correspond with the beginning of the manufacturing process, or may occur as an item is first packaged for delivery. Electronically tagging the item allows for subsequent electronic exchanges of information between the tagged item and a reader, wherein a reader may read information stored on the tag.
Many RFID tags require a high frequency on-chip reference clock signal in order to decode incoming signals as well as generate responses. However, processing variations from wafer to wafer, and even across a wafer, result in one chip having a clock that runs at a different frequency than the clock of another chip. In present manufacturing operations, the frequency of the on-chip clock is calibrated at wafer sort. Particularly, its signal is calibrated against a known frequency from an external source. If the on-chip clock frequency is higher or lower than the external reference frequency, it is adjusted to be within the desired tolerances. Then the setting is permanently programmed into the chip. One drawback of this is that the procedure is performed at wafer sort, which requires extra handling of the chips, and thus increased expense. Another problem is that once the on-chip clock is calibrated, it cannot be recalibrated, at least not without great difficulty. Thus, if conditions change, even temperature, the clock may no longer have the desired frequency.
Methods of calibrating an on-chip clock using the incoming signal frequency as a baseline have also been proposed. However, the incoming signal frequency will typically vary from source to source, and may even be inaccurate itself.